Electroactive polymer devices, in which the polymers switch between redox states to store charge, have been the object of concentrated research over the past several years. As these polymers have the possibility of being switched between their neutral form, a p-type doped oxidized form, and an n-type doped reduced form, a variety of electrode configurations are possible. This has been exemplified by the use of electroactive polymers in supercapacitors, rechargeable storage batteries, and electrochromic devices. S. Gottesfeld, J. P. Ferraris, A. Rudge, and I. Raistrick, Electrochim. Acta, 39, 273, (1994). 2. P. Novák, K. Müller, K. S. V. Santhanam, and O. Haas, Chem. Rev. (Washington, D.C.), 97, 207-281 (1997). 3. P. M. S. Monk, R. J. Mortimer, and D. R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim (1995). 4. S. A. Sapp, G. A. Sotzing, and J. R. Reynolds, Chem. Mater., 10, 2101 (1998). The poly(3,4-alkylenedioxythiophenes) (PXDOTs) have especially useful redox switching properties due to their electron-rich character, which yields especially low switching potentials. L. B. Groenendaal, F. Jones, D. Freitag, H. Pielartzik, and J. R. Reynolds, Adv. Mater., 12, 481 (2000). The parent polymer of this family, poly(3,4-ethylenedioxythiophene), has now been developed to the point of commercialization (Baytron P, Bayer AG) and is used as a stable conducting material in photographic film, tantalum capacitors, and feed-through holes in printed circuit boards. S. W. Schneller and J. D. Petru, Synth. Commun., 4, 29 (1992). In addition, these polymers switch rapidly and efficiently between their neutral and p-doped forms with a minimum of side reactions and long switching lifetimes. As such, they have been heavily investigated for a number of redox devices including electrochromic applications. A. L. Dyer and J. R. Reynolds, in Handbook of Conducting Polymers, 3rd ed., T. Skotheim and J. R. Reynolds, Editors, Taylor & Francis, Boca Raton, Fla. (2008). Polymeric supercapacitors using PXDOTs as the charge carrying layer exhibit excellent reversibility and coulombic efficiency. J. D. Stenger-Smith, C. K. Webber, N. A. Anderson, A. P. Chafin, K. Zong, and J. R. Reynolds, J. Electrochem. Soc., 149, A973 (2002). J. D. Stenger-Smith, J. A. Irvin, D. J. Irvin, T. Steckler, and J. R. Reynolds, Polym. Mater. Sci. Eng., 99, 699 (2008).
It is well known that changes in redox states of electroactive polymers require movement of ions to maintain electroneutrality. Ion choice can affect morphology, stability, and oxidation and reduction potentials. J. A. Irvin, D. J. Irvin, and J. D. Stenger-Smith, in Handbook of Conducting Polymers, 3rd ed., T. Skotheim and J. R. Reynolds, Editors, Taylor & Francis, Boca Raton, Fla. (2008). Ions have traditionally been introduced as a solution of molecular electrolyte, commonly a tetraalkylammonium cation with anions such as ClO4−, PF6−, and BF4−; common solvents include water, acetonitrile, and propylene carbonate. Recently ionic liquids such as 1-ethyl-3-methylimidazolium and Bis(trifluoro methanesulfonyl)imide (EMIBTI) have been investigated as electrolytes, either with or without additional solvent. J. D. Stenger-Smith, C. K. Webber, N. A. Anderson, A. P. Chafin, K. Zong, and J., J. M. Pringle, M. Forsyth, D. R. MacFarlane, K. Wagner, S. B. Hall, and D. L. Officer, Polymer, 46, 2047 (2005). Ionic liquid electrolytes are advantageous due to their wide temperature use window, low volatility, and good electrochemical and thermal stability. W. Lu, A. G. Fadeev, B. Qi, E. Smela, B. R. Mattes, J. Ding, G. M. Spinks, J. Mazurkiewicz, D. Zhou, G. G. Wallace, et al., Science, 297, 983 (2002).
A key component in many electrochromic and other redox switchable devices is the formulation of suitable supporting electrolytes. The electrolyte used in our previous work often consisted of a room-temperature ionic liquid such as EMIBTI. EMIBTI is stable up to 300° C., has an Electrochemical window of 4.3 V, and a melting point of −15° C. J. D. Stenger-Smith, C. K. Webber, N. A. Anderson, A. P. Chafin, K. Zong, and J. R. Reynolds, J. Electrochem. Soc., 149, A973 (2002). 9. J. D. Stenger-Smith, J. A. Irvin, D. J. Irvin, T. Steckler, and J. R. Reynolds, Polym. Mater. Sci. Eng., 99, 699 (2008).
Adding to the challenge to the operation of these devices are conditions required by both civilian and military applications. These conditions typically range from −50 to +50° C. under a variety of humidity conditions. These conditions require that all devices, especially high voltage devices, be hermetically sealed.
The Demand for Low Temperature Operation
With increasing focus on energy production and energy storage, there is an increased need for charge storage devices that operate at a wide variety of temperatures. B. Conway, V. Birss, and E. Wojtowic, J. Power Sources, 66, 1 (1997). 14. C. Arbizzani, M. Mastragostino, and B. Scrosati, in Handbook of Conducting Polymers, 2nd ed., T. A. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds, Editor, Marcel Dekker, New York (1998). There are many batteries that operate well at room temperature or higher, but there are very few that operate below 0° C., and fewer still operate down to −60° C. P. Novák, K. Müller, K. S. V. Santhanam, and O. Haas, Chem. Rev. (Washington, D.C.), 97, 207-281 (1997). C. A. Vincent and B. Scrosati, Modern Batteries: An Introduction to Electrochemical Power Sources, pp. 305-310, John Wiley & Sons, New York (1997). Aside from slowing down the actual kinetics of the charge/discharge reaction (oxidation/reduction, for example), lower temperatures also increase the viscosity of electrolytes, lowering the ability of the electrolyte to transfer charge. Furthermore, at low temperatures, electrolyte mixtures containing ionic liquids may become unstable due to crystallization or co-crystallization of ionic liquid components, phase separation, or supersaturation of components that dissolve into the electrolyte at higher temperatures. Such instabilities lead to degradation of device performance or failure upon extended exposure to low temperatures, even if the mixture has not vitrified. Therefore, there is a great need for stable ionic liquids which can support electrochemical operations at low temperatures.
There are very few electrolytes that are liquid below −60° C. and even fewer room-temperature ionic liquids that are liquid below this temperature. J. Y. Song, Y. Y. Wang, and C. C. Wan, J. Power Sources, 77, 183 (1997). D. Aurbach, Electrochim. Acta, 50, 247 (2004). Adding to the challenge is the fact that these materials must support electrochemical processes at a wide range of temperatures.
It is to be understood that the foregoing is exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.